Request destination for SVG documents

It's unclear what destination value should be used for SVG documents and different browsers seem to be doing different things, at least when it comes to CSP.

https://svg-discrimination.glitch.me/csp.html is loading fine in Chrome and Safari (as long as img-src: 'self' is there), but the SVG is blocked in Firefox.

At the same time, because SVG document resources are internally represented differently than images in Chromium, an as value of "image" for preloads is a mismatch for SVG documents.

Looking at Fetching the document over in SVG-land doesn't mention destination at all.

We should decide what the destination be, and then align spec and implementations on it.
My personal vote would be for a brand new destination (e.g. "svg" or "svg-document"), to make it easier to explain to developers which CSP or as values control these loads.

Start with the Fetch Standard's destination definitions and the linked SVG2 “Fetching the document” section. Compare those with the linked CSP reproduction and Chromium preload issue, then determine an agreed destination for SVG documents and what spec and implementation alignment would count as done.